In the electrophotography, a method which uses a heat roller has been widely employed in order to fix electrostatic latent images which are visualized by a toner. In this method, there has been a desire for a toner having excellent low-temperature fixability (that is, the toner has a low minimum fixing temperature) and a wide fixing temperature range.
If offset is caused on the heat roller, the fixing temperature range is referred to as the difference (T0-TL) between hot offset temperature (T0) and the minimum fixing temperature (TL). Generally, if a polymer having a low molecular weight is used as a binder resin, the minimum fixing temperature and hot offset temperature decrease; moreover, shelf stability lowers. On the other hand, if a polymer having a high molecular weight is used as a binder resin, hot offset temperature increases and shelf stability is improved; however, there is a problem that the minimum fixing temperature increases.
As a means for solving the issue causing the relationship between the hot offset temperature and shelf stability and the minimum fixing temperature is traded-off, a binder resin composition containing a mixture (composition) of a high molecular weight polymer and a low molecular weight polymer, each of which has a different molecular weight distribution, has been known (Patent Literature 1).
As one of other means, there has been disclosed a toner having an inner layer containing a radical polymer resin, a colorant and a release agent, and a toner outer layer containing a grafted polyester resin formed by graft-polymerizing a radical polymerizable monomer on a polyester having unsaturated bonds in a main chain (Patent Literature 2). The above literature discloses the toner having excellent hot offset resistance, an appropriate fixable temperature range and excellent heat-resistant shelf stability.